(i) Field of the Invention
The present invention relates to a disc device aiming at a large memory capacity, particularly to a disc device with a drive system including a highly accurate bearing workable with ease.
(ii) Description of the Related Art
Ball bearings are usually used in spindle motors for information devices. Ball bearings are broadly used because of their low prices and easiness of manipulation. In a ball bearing, however, there is inevitable generation of a vibration and a noise due to a damage or a deformation on a ball or the rolling surface of the ball. Besides, the critical rotational speed is not so high.
In contrast with this, a fluid film bearing (hereinafter called FFB) can afford a far highly accurate rotation with a lower vibration by far in comparison with the ball bearing because it supports a rotating portion in a non-contact manner by an effect of a dynamic pressure generated with the rotation. The FFB therefore has a possibility to meet a high-level requirement.
As a prior art of such an FFB, Japanese Patent Unexamined Publication No. Hei 3-272318 discloses a bearing in which grooves for generating dynamic pressure are formed in a radial bearing surface opposite to a radial receiving surface of a housing and a thrust bearing surface opposite to a thrust receiving surface, and magnetic fluid seals are provided outside of two radial receiving surfaces in the axial direction.
On the other hand, Japanese Patent Unexamined Publication No. Hei 6-200939 discloses a bearing structure using a magnetic fluid as a lubricant, in which radial bearing members are provided respectively at the opening and closing portions of a housing, the end surfaces of the radial bearing members on the side of the opening portion are used as a thrust bearing, the end surfaces of the radial bearing members on the side of the closing portion are used as surfaces for preventing the radial bearing members from coming off, and members for preventing the radial bearing members from coming off are provided on the side of the thrust bearing.
In the FFB, a highly accurate rotation is possible but the bearing rigidity and the bearing load are in proportion because of a fluid dynamic pressure effect. As a result, when the bearing rigidity is increased by decreasing the gap or the width of the bearing, the bearing load increases accordingly. The bearing rigidity of the FFB is lower than that of a ball bearing having the same shaft diameter because the bearing rigidity of the FFB is generated by a dynamic pressure of a fluid. For obtaining the bearing rigidity equivalent to the ball bearing, the bearing load becomes larger 50 to 100% than that of the ball bearing.
Besides, a synthetic oil is used as the lubricating fluid in general but such a synthetic oil has a property that the viscosity varies with temperature, that is, the viscosity decreases with increase in temperature and increases with decrease in temperature. The dynamic pressure effect of a fluid is in proportion to the viscosity of the fluid. As a result, if a bearing is designed in order to ensure a sufficient bearing rigidity at a high temperature, there is a defect that the bearing load becomes very large at a low temperature. In magnetic disc devices used in a wide temperature range, the optimization of the bearing rigidity and the bearing load was a great problem in design.
Because the FFB uses a fluid for lubrication, there is another problem of the leakage of the fluid from the bearing portion. The leakage of the fluid from the bearing portion makes the reliability of the whole device lower by shortening the duration of the bearing and increasing vibrations. Besides, in a disc device, even such a very small amount of leakage as not affect on the bearing performance may cause a serious trouble if the leakage fluid adheres a disc or a head to hinder a recording or reproducing operation.
In the FFB, the performance is not satisfied unless the distance between bearings is kept very minute and highly accurate. For this reason, severe accuracy is required for processing and assembling a shaft and a bearing portion. In case of a device in which a long shaft relative to its diameter is used and a bearing is provided near an end of the shaft, it is hard to satisfy the accuracy due to a deviation of the shaft and a deformation upon assembling.